Liquid crystal displays, as an example of conventional image displays, comprise a liquid crystal display panel, a backlight that is disposed beneath the liquid crystal display panel to supply light to the liquid crystal display panel, and a circuit board used to drive the liquid crystal display panel. The liquid crystal display panel has two transparent glass substrates on which electrodes made of a transparent conductive thin film, an alignment layer, and others are laminated. The transparent glass substrates are assembled at a predetermined distance with their lamination surfaces opposing to each other. Liquid crystal is filled and sealed between these glass substrates. A polarizer is provided outside the glass substrates.
FIG. 32(a) shows a conventional liquid crystal display and a backlight used for it. FIGS. 33 and 34 show other conventional backlights.
As shown in FIGS. 32(a), 33, and 34, the backlight comprises, for example, a light guiding plate 1003 which is formed of a transparent synthetic resin plate for guiding the light emitted from light emitters 1001 away from the light emitters 1001 to uniformly irradiate the entire area of a liquid crystal display panel 1006, fluorescent tubes which are disposed in the vicinity of the end faces of the light guiding plate 1003 along the end faces generally in parallel to the end faces, each fluorescent tube serving as the light emitter 1001, and reflectors 1002 each of which covers the corresponding fluorescent tube to form a light source in cooperation with the latter. The backlight 1000 also comprises a diffusing sheet (not shown) that is disposed on the light guiding plate 1003 to diffuse the beams of light from the light guiding plate 1003, and a reflecting plate 1005 that is disposed under the light guiding plate 1003 to reflect the beams of light leaving the light guiding plate 1003 towards the light guiding plate 1003.
The light guiding plate 1003 has so-called scattering dots 1004 arranged in a predetermined pattern on the lower surface of the light guiding plate. The surface of the scattering dots 1004 serves as a scattering surface. An example of a pattern of the scattering dots 1004 is shown in FIG. 32(b). Thus, the beams of light directed to the scattering dots 1004 are scattered therefrom. Some beams of scattered light leave the light guiding plate 1003 through the upper surface thereof. In addition, the beams of light that enter other areas than the scattering dots 1004 on the lower surface of the light guiding plate 1003 propagate through the light guiding plate 1003 by a series of multiple scatterings caused by internal reflection according to the angle of incidence.
Changing the distribution of the scattering dots 1004 results in varied luminance distribution on the light exit surface (upper surface) of the light guiding plate 1003. The beams of light that are directed from the light source to the light guiding plate 1003 propagate through the light guiding plate 1003 by a series of internal reflections, with a percentage of light transmitted to the outside. The amount of the light is larger at a position near the light source and becomes smaller when it travels away from the light source. Taking this into consideration, as shown in Japanese Utility Model Laid-Open No. 60-76387, the area ratio for the scattering dots 1004 on the lower surface of the light guiding plate 1003 may be made low on the side of the light source. This decreases the percentage of light directed towards the lower surface of the light guiding plate 1003, that is scattered by the scattering dots 1004 and that leaves the light guiding plate 1003 through the upper surface thereof. The area ratio may be increased according to the distance away from the light source. The percentage of light that is scattered by the scattering dots 1004 and that leaves the light guiding plate 1003 through the upper surface there of increases at a location distant from the light source. Consequently, the ratio of the amount of light that comes out at any point to the amount of light that comes out from the whole light guiding plate 1003 through the upper surface thereof (hereinafter, referred to as a “light intensity distribution”) becomes equal, which improves uniformity of the luminance.
The liquid crystal display with the above-mentioned backlight 1000 can improve uniformity of the luminance on a display screen of the liquid crystal display panel 1006.
Transmissive liquid crystal displays that are in the current mainstream require illumination from behind using a backlight, as described above. However, the beams of incident light are random beams that are polarized in various directions. Almost half of the beams is absorbed by the polarizer on the incident side. This results in a low light utilization efficiency. In view of this, a prism sheet is used that effectively condenses the beams of the diffused light from the backlight within a viewing angle to enhance face-up luminance.
FIG. 35 shows a liquid crystal display with such a prism sheet. In FIG. 35, the beams of light from upper and lower light sources 1002 enter the light guiding plate 1003. A reflecting sheet 1005 is used to return the beams of light that leak from the light guiding plate 1003 into the light guiding plate 1003 for effective use. The beams of light that are scattered by a diffusing sheet 1031 are condensed via a prism sheet 1033 and then enter a liquid crystal element 1035. Polarizers 1034 and 1035 are disposed in front of and behind the liquid crystal element 1035 with their polarizing axes perpendicular to each other. The prism sheet 1033 provides a different transmittance depending on the polarization direction of the incident light. This is determined according to the relation between the angle of a concave/convex portion on the surface of the prism sheet 1033 and the direction of oscillation of the incident light. A characteristic curve of it is as shown in FIG. 26. FIG. 26(a) shows a change in transmittance as a function of an angle of incidence in the up-and-down direction relative to the prism sheet 1033, i.e., in the direction perpendicular to an edge line direction 1045 of the prism sheet 1033. On the other hand, FIG. 26(b) shows transmittance as a function of an angle of incidence in the edge line direction 1045 of the prism sheet 1033. The results given in FIG. 26 indicate that the transmittance varies between p-polarized light and s-polarized light. The transmittance for p-polarized light is high in either directions on the face-up direction, that is, in the direction of a viewing angle range between −10 and +10 degrees. Therefore, when the face-up luminance is an important factor, the luminance can be enhanced by predominantly using p-polarized light. Japanese Patent Laid-Open No. 2000-122046 discloses such an attempt to fix a transmission axis of the polarizer on the incident side according to the light in a given oscillation direction that passes through the prism sheet, thereby improving the light utilization efficiency of a liquid crystal panel.
However, in the above-mentioned conventional backlight, some beams of light that come out of the light source 1002 do not hit the scattering dots 1004 and leave through an opposite end face instead, as depicted by dotted line in FIG. 34. The beams of light become lost beams that leak outside the light guiding plate 1003. In this case, a reflection tape may be provided on the opposite end face to return the light to the light guiding plate 1003. Recent light sources 1002 are, however, often disposed on both ends of the light guiding plate 1003 for higher luminance, as shown in FIGS. 32 and 33. In this case, no reflection tape can be provided. Some beams of light that reach the opposite end face are reflected by the fluorescent tube 1001 and the reflector 1002 and re-enter the light guiding plate 1003 for subsequent use, while other beams do not re-enter it. They are lost. According to the results of experiments and simulations that the present inventors have made, almost half of the beams of light that reaches the opposite end face of the light guiding plate 1003 is lost. Approximately 18% of the light that leaves the fluorescent tube 1001 goes through to the opposite side, about half of which is lost. In order to reduce the penetration of the beams of light through the light guiding plate, the scattering dots 1004 may be arranged more densely. The scattering dots 1004 are typically formed by printing. However, the density of the scattering dots 1004 has an upper limit by safety considerations in manufacturing. More specifically, an attempt to provide denser scattering dots 1004 cannot result in printing of the dots having a designated area because adjacent scattering dots 1004 are coalesced into one. The degree of coalescence varies each time of printing, which interferes stable fabrication. Thus, a certain spacing should be required between the adjacent scattering dots 1004. This can be expressed by the ratio of the total area of the scattering dots 1004 to the area available for the scattering dots 1004 (hereinafter, referred to as an “area ratio”). The upper limit of it was 80%. The scattering dots 1004 that can be printed in a stable manner has a lower limit as well. The lower limit was 20% in terms of the area ratio. Thus, the above-mentioned backlight 1000 has the upper limitation of the area ratio for the scattering dots 1004. This causes the above-mentioned penetration of beams of light through the light guiding plate 1003. The beams of light from the light source 1002 cannot be fully utilized.
To cope with such problems, Japanese Patent Laid-Open No. 8-146231 discloses a configuration in which a diffusing material is incorporated into a light guiding plate to scatter the beams of light for display use. However, the approach of incorporating the diffusing material into the light guiding plate to scatter the beams of light can only improve the scattering efficiency within the light guiding plate. It is difficult to control, at the same time, the luminance distribution on the light exit surface of the light guiding plate.
Recent liquid crystal displays have been in widespread use as a display monitor for PCs (personal computers). In addition, they have found applications as a liquid crystal television on which people can watch, for example, films with motion pictures. Liquid crystal displays for use in display monitors have been developed with higher resolution and higher luminance. Display monitors are mainly used for displaying text strings and drawing pictures. Accordingly, uniform luminance is required over the entire area of the display screen. In fact, as to the luminance distribution over the entire area of the display screen, the luminance at peripheral portions is at least 80% of the luminance at the central portion.
On the other hand, conventional televisions using a CRT (hereinafter, referred to as a “TV”) typically have a higher luminance at the central portion of the display screen. The luminance at peripheral portions may be on the order of 50% of it. People usually look at the central portion of a motion picture such as a film. A large difference in luminance of the luminance distribution on the display screen seldom makes unnatural impression. Rather, the screen appears brighter when the luminance is higher at the central portion even though the luminance at peripheral portions is reduced.
As apparent from the above, displays have their suitable settings of display characteristics such as the luminance distribution according to specific applications such as a display monitor or a TV. In the field of the liquid crystal display, those available for AVPCs (audio video personal computers) have been developed recently that can function as both the display monitor and the TV. The liquid crystal display uses the illumination from the backlight for displaying images. Accordingly, it is necessary to distribute the luminance by varying the light output characteristic of the backlight in order to provide varied luminance distributions.
In typical edge-light type backlight systems using a light guiding plate, the distribution of the luminance is controlled by using the scattering dots that are provided on the lower surface of the light guiding plate, as described above. However, the scattering dots are formed by printing into a fixed pattern. Random variations cannot be achieved for settings of the luminance distribution of the backlight. Thus, no liquid crystal display with a conventional backlight can change the settings of the luminance distribution according to the utility of it.
The above-mentioned conventional liquid crystal displays are typically lower in luminance than CRTs. A higher luminance is desired accordingly. In order to provide a higher luminance, the output of a light source should be increased. In this event, the half of the beams of light from the light source 1002 is absorbed by polarizer on the incident side 1034 in the conventional liquid crystal panel 1006 having the configuration as shown in FIG. 35. The larger amount is absorbed with a larger output of the light source 1002. The uniformity is deteriorated due to thermal contraction of the polarizer 1034 by absorbed light. This may cause a problem of irregularities in black display.
In addition, an ratio of beams of light absorbed that has a low transmittance for p-polarized light and s-polarized light cannot be neglected because the non-polarized random light enters the prism sheet 1033. Consequently, deterioration of light condensing characteristic caused by deformation of the prism sheet 1033 due to heat would be a problem.
Furthermore, in FIG. 35, the polarizing axis of the polarizer is tilted about 45 degrees to the liquid crystal element 1035 for the following reason. TN liquid crystals that are widely used for the liquid crystal display panel 1006 have varied contrast viewing angle characteristics. It is wider in the horizontal direction and narrower in the vertical direction. The transmission axis of the polarizer is tilted 45 degrees to adjust the contrast viewing angle characteristics. Accordingly, the contrast viewing angle characteristics would be deteriorated when the transmission axis of the polarizer on the incident side is determined to suit to the light in a particular oscillation direction that passes through the prism sheet.